1. Field of the Invention
This invention relates to a charge transfer device having plural registers arranged in parallel with one another.
2. Description of the Prior Art
In a charge transfer device in general, such as a CCD, a region called a register for charge transfer is formed on its chip. A transfer electrode for potential control is formed in a side-by-side relation with respect to the register of the charge transfer device, and a channel region operating as the charge transfer route is formed below the transfer electrode.
The CCD solid state imager is a device consisting essentially of such a charge transfer mechanism and an annexed sensor for opto-electric conversion. Above all, in a color solid state imager, there are formed filters of different color transmission types, and sensors associated with the different colors. For example, in a color linear sensor, there are formed one or more rows of sensors for outputting signals of different colors. In a device in which the sensors associated with the different colors are arrayed sequentially in a row, a register is provided in parallel with the sensor row. Signals of the different colors are collectively transferred to the register from the sensors so as to be allocated and read out in the order in which they are output. In a device in which plural rows of sensors are formed, a register is provided for each sensor row and outputs an associated one of the three color signals.
The number of the registers in the charge transfer device may not be one, and there may be cases wherein a plurality of registers are formed. When a plurality of registers are provided in parallel with one another, it becomes necessary to transfer electrical charges across the registers.
FIG. 1 shows to an enlarged scale the region between the adjacent registers of a conventional charge transfer device. A first polysilicon layer 101 is formed along the direction H. This first layer of polysilicon functions as a transfer gate between the registers. A second layer of polysilicon 102 and a third layer of polysilicon 103 are alternately formed at a predetermined pitch along the direction H shown in FIG. 1. The patterns of these polysilicon layers 102, 103 are extended in the direction V in FIG. 1 with obliquely extending portions over the polysilicon layer 101. The polysilicon layers 102, 103 function in common as the transfer electrodes for the first and second registers 104, 105 and are operated by two-phase driving. When the charges are transferred in the direction V, that is, across the registers 104, 105, a pulse is transmitted to the first polysilicon layer 101, and the electrical charges in the first register 104 are transiently stored in a lower portion of the first polysilicon layer 101. The charges thus stored in the first polysilicon layer 101 are then transferred to the second register 105.
However, with a color linear sensor, should the sensors associated with the different colors be arrayed sequentially in one sensor row, signal charges read into the registers may be degraded as they are being transferred in the registers, so that color mixing may occur in the produced image. Should a plurality of rows of registers be formed in association with a plurality of rows of sensor rows, the sensor rows may be unavoidably different in their positions to increase the load in signal processing.
On the other hand, should electrical charges be transferred across a plurality of registers, as shown in FIG. 1, the first layer of polysilicon 101 need be formed in a long pattern length along the direction H shown therein. However, a linear sensor or the like having a longer length along the direction H has an increased resistance. Also, when the charge transfer in the direction H is to be achieved by two phase driving, electrical charges need be transiently stored in a lower portion of the first layer of polysilicon 101 with consequent deterioration in the transfer efficiency.